In telecommunications systems, power control is important for the prevention of signal fading, shading, near-far problems and co-channel interference. Power control is particularly necessary in situations in which system users are located both near and far from a base station. Signals should arrive at the base station receiver with equal power or else the signals arriving with a lower power would be covered by the higher-power signals. Transmitters located far away should thus transmit the signals at a higher power than transmitters close to the base station. In addition, the signals should be transmitted at an as low power level as possible that would, however, be sufficient to meet the quality requirements set for the signals.
In prior-art CDMA systems (e.g. IS-95, CDMA-PCS, UMTS, CDMA2000), the transmission power of the terminal and base station is controlled for instance by first measuring the signal-to-interference ratio of the transmitted signal and determining by means of it and other reference values a power control command for adjusting the transmission power at each time. The terminal increases or decreases its transmission power according to instructions provided by the base station in accordance with the power control commands it receives in each time-slot. In the IS-95, CDMA-PCS and CDMA2000 systems, there are two power control commands: increase and decrease. In the UMTS system, there are three (in theory four) power control commands: increase, decrease and maintaining the power. In the base station, a reference level is determined for instance on the basis of the ratio between bit energy and interference energy and the bit error rate, and the reference level is used to control power, i.e. to increase or decrease power as necessary. The base station then transmits to the terminal a command to either increase or decrease transmission power, whereby the terminal increases or decreases its earlier transmission power level by a predefined quantity, usually one or two decibels at a time.
Drawbacks with the prior-art solutions include the fact that since power can only be increased by a predefined quantity, generally one or two decibels at a time, with one power control command, it is not possible to react to rapid changes in channel conditions. Such a situation arises in a rapidly fading channel, wherein power should be quickly increased or decreased large quantities at a time to obtain an optimum power level. A problem with the prior-art solutions is thus a long transmission-power change time, in addition to which, the transmission power level also varies continuously.